1. Field of the Invention
This invention relates to prosthetic joints generally, and more particularly to a rotating/sliding constrained prosthetic knee replacement for a dysfunctional knee.
2. Prior Art
Referring now to prior art knee endoprostheses, there are basically two types of prosthetic replacement knees known generally as constrained and unconstrained knees. An example of a prior art constrained or hinged knee is shown in U.S. Pat. No. 4,219,893 to Douglas G. Noiles. An embodiment of the Noiles invention is manufactured and sold by Howmedica, Inc. of Rutherford, N.J. An example of an unconstrained or floating meniscal bearing knee is disclosed in Buechel et al U.S. Pat. No. 4,340,978. An embodiment of the Buechle invention is manufactured and sold by Zimmer, Inc. of Warsaw, Ind. Preferably, the bearing elements of both types of knees are manufactured with high density polyethylene such as that disclosed in Zachariades U.S. Pat. No. 4,587,163 developed by Polteco Inc. of Alameda, Calif. because of its superior wear resistant characteristics. Both classes of prior art prosthetic knees have had problems often resulting in failures requiring additional surgery and repair or reconstruction.
In particular, pre-existing constrained knees have often resulted in early failure as a result of hinge constrainment. The degree of rotation was limited to either only one plane or a very small arc causing a loosening and failure of the connection points between the prosthesis and the tibia or femur. Also, as shown in U.S. Pat. No. 4,219,893, very little flexibility was possible in the shape of the patello-femoral interfaces because of the requirement to maintain congruent patello-femoral contact over the range of motion of the knee. As a result, patello-femoral tracking problems became commonplace.
It was necessary to use a large circumference when resurfacing allografts resulting in problems with soft tissue necrosis and/or patello-femoral tracking problems as described above. Furthermore, most implants were custom devices since they had to be specially made to fit a particular patient's size and thus required excess manufacturing time and unnecessary delays. None of the prior art devices of either class were capable of being made in "modular" form, which simplified procurement and inventory problems associated with custom devices.
An additional, significant problem with prior art constrained knees results from the fact that the range of motion prevents the normal A-P movement of the inferior end of the femur relative to the superior end of the tibia. This "sliding" movement is necessary in order to maintain the full range of motion desired in a prosthetic device and to approximate normal human kinematics.
Both constrained and unconstrained prosthetic knees suffer from some of the same deficiencies but also include additional problems. In particular, it has been shown that such prior art prostheses have poorly designed patello-femoral interfaces in that they do not provide reasonable congruent patello-femoral contact or sliding engagement over any appreciable range of knee motion. These prior art prostheses typically produce contact stresses which result in yielding and fatigue of the plastic bearing surface typically present in such prostheses. This result is caused by the fact that the bearing surface of the femoral component, over which the patella prosthesis must pass, generally has several regions or segments of differing shape. For example, there is typically a fairly long, singly curved segment blending into a first doubly curved segment blending again into a second, and different, doubly curved segment. Thus, when the patella prosthesis goes through its excursion over the femoral articular flange, the patella prosthesis undergoes a variety of contact conditions, namely, substantial portions of line contact, portions of point contact, and perhaps limited portions of area or congruent area contact. As is known, line contact and point contact conditions generally produce high contact stresses which produce yielding and possible wear of the polyethylene portion of the prostheses. Hence, the extended wear life needed for successful prosthetic implantation is not realized.
Referring next to typical prior art tibial-femoral knee prostheses, prostheses which allow axial rotation and A-P motion in addition to flexion-extension motion have incongruent contact (usually theoretical point-contact) between the femoral and tibial bearing surfaces, have been found to produce excessive contact stresses leading to deformation and/or early wear and undesirably short prosthetic life. Also, wear products have been shown to produce undesirable tissue reactions which may contribute to loosening of the prosthetic components.
Those prior art knee prostheses which do provide congruent or area bearing contact fail to provide the needed axial rotation, or when cruciates are present the needed anterior-posterior motion. This lack of axial rotation and anterior-posterior motion has been found clinically and experimentally to result in deformation and loosening of the tibial components, and such prostheses now appear to be falling into disuse.
Current prostheses of the dislocatable cruciate retaining type, such as the Geomedic knee replacement shown in U.S. Pat. No. 3,728,742 to Averill et al., that produce area contact provide only one axis of rotation relative to the femur for the flexion-extension motion. Normal flexion-extension is, however, characterized by apolycentric flexion-extension motion where rotation relative to the femur occurs about many axes. This polycentric motion, which results from the action of the cruciate ligaments and condylar shape, allows for more efficient utilization of muscle forces by providing a posterior shift of the axis when effective quadriceps action is important and an anterior shift when hamstrings effectiveness is important. Furthermore, in the human knee it is this action and the A-P shift, and the shape of the posterior condyles, which influence this motion so as to allow full flexion capability for the knee. Failure to provide appropriate knee geometry inhibits, when cruciate ligaments are present, this natural motion and thus tends to restrict muscle effectiveness and inhibit flexion. These restrictions tend to increase both loading on the prosthesis (which increases wear or likelihood of deformation or breakage) and loading between prosthesis and bone (which increases the possibility of component loosening).
It has been found that loosening problems result from the direct attachment of plastic prosthetic components to bone through the use of relatively brittle cement that is weak in tension. Specifically, it has been demonstrated that even relatively thick plastic components when loaded in a normal fashion produce undesirable tensile stresses in the acrylic cement commonly used to secure such plastic components to bone. Such loading tends to produce bending of the plastic component which causes the ends of the plastic component to lift away from the bone, thereby subjecting the bone-cement attachment to tension. As is known, cement has very poor tensile fatigue properties. The bone to which the plastic prosthesis is cemented also appears to be adversely affected by tensile loads. Accordingly, these combined effects contribute substantially to prosthetic loosening problems and, specifically, it has been noted where clinical failure due to loosening occurs in a knee prosthesis that is almost always the plastic prosthesis component which loosens.
Another prior art prosthesis problem exists with regard to knee endoprostheses for implantation in those cases wherein the cruciate ligaments are functionally absent but where the collateral ligaments are functional or at least reconstructable. In the absence of cruciate ligaments, the prosthetic replacement must provide anterior-posterior knee joint stability so as to replace that stability otherwise provided by the cruciates. Until recently most such cases were treated by a constrained type knee prosthesis which may suffer from the loosening problems described above caused by the stresses described above. Necrosis of the bone, caused by altered mechanical bone stresses, is also a problem with the prior art constrained knee prostheses.
Where the cruciate ligaments are present, most surgeons would prefer their retention, since they provide important internal stabilizers and, together with the condylar geometry of the femur and tibia, control the rotation axis of the knee. Furthermore, these ligaments provide anterior-posterior stability. Thus, it is desirable to reserve the cruciate ligaments, even though reasonable stability can be provided by a properly designed full platform type prosthesis.
In addition, the action of the cruciate ligaments produces a shift in the rotation axis of the knee which may result in more efficient muscle utilization. Thus, preservation of these structures may provide better physiological function after knee replacement.
It is not, however, clear that the physiological advantages gained in retaining the cruciates outweigh the disadvantages of the design compromises, such as increased bearing surface incongruency and reduced tibial prosthesis bearing area, required to retain these ligaments. Thus, the desirability of retaining the cruciate ligaments in the cases of unconstrained knee replacement is not well established.
A recent unconstrained knee concept, the New Jersey knee, appears to provide a partial solution to the problem of overconstraint while maintaining congruency by the use of mensical floating elements. Unfortunately, this knee suffers from several design problems which appear to limit its usefulness.
The present invention, the Finn Knee, utilizes new concepts combined in an improved low profile design in order to avoid some of the anticipated difficulties of the prior art design.